Optical fibres and integrated optical waveguides are today applied in a wide range of applications within areas such as optical communications, sensor technology, spectroscopy, and medicine. These waveguides normally operate by guiding the electromagnetic field (the light or the photons) through a physical effect, which is known as total internal reflection. By using this fundamental effect, the propagation (or loss) of optical power in directions perpendicular to the waveguide axis is reduced.
In order to obtain total internal reflection in these waveguides, which are often fabricated from dielectric materials (in optical fibres) or semiconductors (in integrated optics), it is necessary to use a higher refractive index of the core compared to the refractive index of the surrounding cladding.
Today the preferred signal transmission medium over long and medium distances is the optical fibre, and total internal reflection is, consequently, a physical property, which has been known and used in technological development for decades. During the past ten years, however, the development within the area of new materials has opened up the possibilities of localisation of light or control of electromagnetic fields in cavities or waveguides by applying a completely new physical property—the so-called photonic band gap (PBG) effect.
The PBG effect may be introduced by providing a spatially periodic lattice structure, in which the lattice dimensions and applied materials are chosen in such a way that electromagnetic field propagation is inhibited in certain frequency intervals and in certain directions. These PBG materials have been described in one-, two-, and three-dimensional cases in the scientific literature and in several patents (see for instance U.S. Pat. No. 5,356,215, U.S. Pat. No. 5,335,240, U.S. Pat. No. 5,440,421, U.S. Pat. No. 5,600,483, U.S. Pat. No. 5,172,267, U.S. Pat. No. 5,559,825).
A specific class of components, which makes use of such periodic dielectric structures, are the optical fibres (or waveguides), in which the periodic variation appears in directions perpendicular to the waveguide axes, whereas the structures are invariant along the waveguide axes.
Within recent years, especially researchers from University of Bath, UK, (see e.g. Birks et al., Electronics Letters, Vol.31 (22), p.1941, October 1995) have presented such fibres realised by having a silica core surrounded by thin, parallel, and air-filled voids in a silica-background material, and organising the air-filled voids in a triangular structure in the cladding region of the fibres. These optical fibres have demonstrated interesting propagation properties compared to standard optical fibres utilising total internal reflection.
It may be a problem or disadvantage of this particular realisation of optical fibres with periodic dielectric cladding regions that careful stacking of either hexagonal glass tubes (with central holes) or direct stacking of thin circular tubes is required. These tubes have been arranged in a triangular structure in a perform, where after the perform is drawn into an optical fibre. Although these fibres according to the reports in the international literature show quite specific and new optical properties, one of the problems has been that the core of the fibre could only be formed by introduction of a glass rod without a central hole.
It is a further disadvantage that even these new fibres have core areas with higher refractive index than the average refractive index of the surrounding media, and their waveguiding effect may, consequently, also be described by conventional total internal reflection. This naturally has the consequence that for applications within areas such as optical sensors, where it may be of specific interest to be able to localise optical fields in and around areas with low refractive indices (e.g., around air-filled channels), the presently known fibres may not be used directly.
It is a still further disadvantage that, due to the triangular cladding structure, the PBG of the structures described by Birks et al. are not optimised for guiding electromagnetic radiation using the PBG effect.
U.S. Pat. No. 5,802,236 discloses micro-fabricated optical fibres having a core and a cladding region, wherein the cladding region comprises a multiplicity of spaced apart cladding features that are elongated in the direction of the fibre. The effective refractive index of the cladding region is less than the effective refractive index of the core region. Furthermore, the elongated features in the cladding are arranged in a non-periodic structure.
It is a disadvantage of the micro-fabricated optical fibre disclosed in U.S. Pat. No. 5,802,236 that due to the high-index core region the waveguiding characteristics are based on traditional total internal reflection of the electromagnetic radiation guided in the core region.
WO 99/00685 discloses a large core photonic crystal fibre (PCF) comprising a cladding having a triangular periodic structure. The core region may be either a high-index or low-index region having a diameter of at least 50 μm. In a preferred embodiment the core region may be as large as 50 μm in diameter. With such a diameter, the fibre is capable of transmitting high powers, whilst maintaining single-mode operation.
It is a disadvantage that the triangular cladding structure disclosed in WO 99/00685 will not, using realistic manufacturing parameters, provide a sufficient PBG so as to effectively confine visible or NIR electromagnetic radiation within the core region of the fibre.
It is a further disadvantage of the structure disclosed in WO 99/00685 that in order to obtain a sufficient PBG, the dimensions of the features forming the triangular periodic structure must be very large. Such large features, e.g. air-filled holes, make the structure difficult to manufacture and inconvenient for practical applications.
It is an object of the present invention to provide a new class of optical waveguides, in which waveguiding along one or more core regions is obtained through the application of the PBG effect.
It is a further object of the present invention to provide a two-dimensional lattice structure capable of providing complete PBG's for realistic manufacturing parameters.
It is a still further object of the present invention to provide low-index core PBG waveguiding structures.
It is a still further object of the present invention to provide a PBG structure, which is easy to manufacture.